Typical Schematic Cross-section of Winds During Advection Fog
Season Along California Coast

(Click on Image To See Larger Version)

This highly schematic diagram shows the conceptual highlights of the
circulation pattern during the summer associated with advection
fog along the California coast on a typical summer afternoon. The temperatures show surface temperatures.

Since the typical dew point temperature is about
56F for the incoming air, moisture condenses when the air passes across the
California current. Surface air there is cooled to the dewpoint, but
only in the air immediately in contact with the sea surface. Some mixing
does occur so that this marine layer is about 500 ft
to about 1000 ft thick.

The air is drawn inland through gaps or low passes through the Coast
Range into the Central Valley, where the air is heated from beneath. The
fog evaporates (dissipates) as the difference between the temperature and
the dewpoint gets large again.

Air in the Central Valley ascends. However, since the depth of
the atmosphere affected by this whole differential heating and cooling is
only around 4000 feet or so, this ascending air does not get cooled to the
dewpoint. Hence, no clouds form over the Central Valley.

Notice that the marine layer is too shallow to be involved in the production
of thunderstorms in the Sierra Nevada.